.center {
    position: absolute;
    top:0;
    right:0;
    bottom:0;
    left:0;
    margin:auto;
}
#root {
    width: 300px;
    height: 300px;
    background-color: pink;
    position: absolute;
    top:0;
    right:0;
    bottom:0;
    left:0;
    margin:auto;
}
#outer {
    width: 200px;
    height: 200px;
    background-color: blue;
}
#inner {
    width: 100px;
    height: 100px;
    background-color: green;
}